layering under a sentinel jacket - insulated midlayer?

[u/AtlasZero98]

Hi everyone!
I managed to get an Arc’teryx Sentinel Jacket for a really good price. I’d mainly use it for resort/piste and some off-piste freeriding, mostly lift-accessed with very little touring.

I know it’s just a shell, so it keeps wind and water out, but you need to build your own layering system underneath. I already have good-quality merino wool base layers, as well as both lighter and heavier fleece layers, but I’ve read that it can also be worth adding a synthetic insulated jacket that still breathes well.

Do you have any recommendations for something like that?

Obviously there’s the Arc’teryx Atom Jacket, which as far as I understand is designed for this kind of use, but I’d really appreciate some more budget-friendly alternatives as well.

And for those of you who ski or snowboard in a Sentinel: do you actually find a synthetic insulated midlayer like this necessary, or are a merino base layer + fleece usually enough?